This invention relates to electronic circuitry and more particularly to power filters which are used to improve the waveform quality in multiple phase power systems.
Some AC power supply applications require high quality sine wave outputs. For example, in aircraft electrical systems, specifications call for a 4% maximum total harmonic distortion, a 4% maximum voltage imbalance for a 2/3 per unit load unbalance, and a 0.9% maximum modulation. These requirements can be difficult to meet due to the high source impedances of typical aircraft generators and other characteristics of these sources.
Active power filters have been designed for insertion between such electrical power sources and a load to generate a synthetic ripple which opposes and effectively eliminates an electrical ripple generated by the power sources. One such active power filter is disclosed in U.S. Pat. No. 3,825,815, issued July 23, 1974 to Gyugyi et al.
Typical power active filters operate with separate reference waveforms for individual phases. Those filters do not force a balanced output and therefore do not necessarily minimize the volt-amps handled by each power active element. As a result, one reactive element could be required to produce higher volt-amps than necessary. It is therefore desirable to produce a power active filter which minimizes the volt-amp rating requirements of the individual series connected power active filter elements.